Ultrasonic imaging has been widely used in ultrasonic image guided interventional operation and reestablishment of three-dimensional volume data. However, a common two-dimensional ultrasonic probe does not have peripheral positioning equipment, so ultrasonic image data cannot be directly used in operation. Therefore, a corresponding transformation relation must be established between a locating device and an ultrasonic imaging plane. Calibration of an ultrasonic probe is a process of confirming the transformation relation of transforming a coordinate system of a position sensor fixed on the ultrasonic probe into a coordinate system of a two-dimensional ultrasonic imaging plane.
In a calibration algorithm of the ultrasonic probe, design and making of calibration model are crucial. The quality of the design of the model directly affects complexity of calibration operation, clearness of imaging of calibration feature points, convenience of extraction of the feature points and precision of follow-up calibration solving. Typical calibration models for ultrasonic probes can be divided into the following two kinds: phantom-based calibration models and stylus-based calibration models.
In single-point phantom calibration, a round object (as shown in FIG. 1a) or an intersection point of intersection lines (as shown in FIG. 1b) are scanned and imaged at multiple angles, and the object is separated from a scanned image, and is generally regarded as the origin of coordinates of a phantom object for solving. Calibration precision of the kind of method depends on locating accuracy to an object of a feature point on an ultrasonic image, and the ultrasonic image plane requires exactly passing through the center point of the object of the feature point. Several intersection lines form several imagable round intersection points in the multi-point and intersection line phantom, which also requires the ultrasonic image plane to pass through the plane with the intersection points. The several intersection points often have a three-point collinear or three-point coplanar triangular relation (as shown in FIG. 1c), and a calibration equation is solved by utilizing such geometrical-restriction relation. Calibration thinking of a two-dimensional-shaped phantom is similar to that of a multi-point phantom, which scans the intersection points (as shown in FIG. 1d) of the intersection lines instead of the geometric angular points of a two-dimensional planar object, and generally shows higher brightness value in terms of the scanned images. A three-intersection-line phantom consists of three intersection lines which are perpendicular in a pairwise manner (as shown in FIG. 1e). As the original intention of the design of the phantom, a coordinate system consisting of the three intersection lines is used as a local coordinate system of the phantom, so that an ultrasonic scanning plane is unnecessarily to be perpendicular to a calibration phantom of an ultrasonic probe, and can simplify scanning operation relatively.
As the method of calibration of a surface phantom, a flat plate capable of imaging clearly in ultrasound is fixed at the bottom of a water tank or in the water tank and is used as the calibration phantom of the ultrasonic probe; the image of the phantom in ultrasound is a straight line, so that follow-up image features (straight line) are easier to be extracted, and the quantity of points on the straight line can be quite rich for the solving of the calibration equation. N-shaped targets (as shown in FIG. 1f) are formed by an N-shaped phantom through a layer or multiple layers of nylon wires; when an ultrasonic plane passes through the N-shaped targets, each N-shaped target wire produces three bright spot feature points on the image. After coordinates of three bright spots of each N-shaped target wire are manually recognized and picked up, a three-dimensional coordinate value of an intersection point of the corresponding N-shaped target and the imaging plane in a design coordinate system can be solved as per the ratio of the distance between the left bright spot and the middle bright spot to the distance between the right bright spot and the middle bright spot according to the design constraint of the model.
In recent years, because N-shaped phantoms are simple to be made and speedy and convenient to be scanned, they have been widely used. However, a sound field of an ultrasonic probe is diffused with increase of scanning depth; and the ultrasonic imaging plane is not an ideal geometric plane. An imaging plane with certain thickness is intersected with a linear target; projection of a transmitting target on an ideal imaging plane cannot be comparable to a point, thus, a linear and even arc-shaped bright spot feature point is formed. However, the light spot has quite large error and uncertainty when the coordinates of a mark point are manually or automatically picked up, which results in larger error in calculation of reestablishment of three-dimensional coordinates of the N-shaped targets, loss of co-planarity which originally existed in the process, and reduction of calibration precision.